It looks like the DeltaDS parameter of the BeforeUpdateRecord event contains only information to the nested dataset when the call occurs for the details.

It would be nice if I could do something like:

DeltaDS.ParentDS.FieldByName('FIELDNAME').NewValue.

Edit:

When using nested datasets the BeforeUpdateRecord event is called twice, once for the master and once for the detail (if we have one record of both). When the event is called for the detail, is there a way to access the master information contained in the DeltaDS ?

We can't access the data of the master CDS at that moment as the changes are not already merged. I hope this is not adding more confusion.